The present invention relates to a raw material supply device for supplying a gaseous glass raw material (frit) to a reaction device of producing a preform (mother material) used in the production of an optical fiber.
For producing a preform for optical fiber of a type used for communication, a MCVD method, a VAD method and like methods have been employed. Such methods are described in "Techniques of Optical Fiber for Communication on Survey of The Patent Journal" (compiled by the Japanese Patent Office, published by The Invention Association of Japan) Apr. 20, 1982, pp. 89-122. According to any one of these methods, in order to obtain glass of high transparency and high purity, a supply of liquid glass raw material (frit) of which silicon tetracloride serves as the main raw material and including germanium tetracloride or the like as a dopant, is put into a raw material container. Into this container, a carrier gas is introduced which evaporates the raw material. The carrier gas including the gaseous raw material in the saturated or nearly saturated state is applied to a reaction device for production of the preform. Within the reaction device, the gas is oxidized, resulting in an accumulation of a grained oxide production, namely, the preform.
A conventional raw material supply device for implementing the above process was constructed as shown in FIG. 1. In this device, a liquid glass raw material 2 is charged in a raw material container 1 and this container, in turn, is received in a temperature-controlled bath 3 employed to maintain the temperature of the material container and its contents uniform. The raw material container 1 is secured to the bottom of the temperature-controlled bath 3 directly or indirectly via supporting stands 4. A carrier gas 5' is introduced into the raw material container 1 through a pipe 5 and a flow control device 6 (such as a mass flowmeter). The carrier gas 5', which is supplied in a bubbling state, absorbs the raw material gas, becoming saturated or nearly saturated therewith. The gas, which is now a reacting gas 7', is passed from the raw material container 1 through a pipe 7 to a reaction device (not shown). In the above-described process, the carrier gas need not necessarily be supplied in the bubbling state. It is possible merely to introduce the gas into a space above the liquid level and cause the same to be mixed with the raw material gas evaporated from the liquid surface.
When the liquid raw material decreases in volume as a result of consumption, a shut-off valve 8 provided on a supplement pipe 9 coupled to the raw material container 1 is opened so that a supply of liquid raw material 9' is passed under pressure to the raw material container 1 from a service tank (not shown), thereby to recharge the liquid raw material. In the foregoing system, however, the pressure and temperature inside the raw material container can vary greatly at the time of liquid supplement, resulting in a difficulty that the desired reaction will not proceed as required, making it necessary to halt the operation of the reaction device until the raw material supply device recovers its normal state. As the operating time of the reaction device becomes longer, as has been the tendency in recent systems, and as the velocity of reaction, that is, the rate of consumption of the raw material, increases, the operating rate of the reaction device is actually lowered due to the necessity of liquid supplement. It is not possible to alleviate this problem significantly by an enlargement in the volume of the raw material container. Moreover, other difficulties such as leakage of liquid caused by the increase in the frequency of the liquid supplement occur.